In the automotive sector and in buildings, glazing systems made from compositions containing transparent thermoplastic polymers such as e.g. polycarbonate offer many advantages over conventional glazing systems made from glass, such as e.g. greater break resistance or weight savings. In the case of automotive glazing systems, they provide greater passenger safety in the event of traffic accidents, and the weight savings reduce fuel consumption. Finally, transparent thermoplastic polymers and compositions containing transparent thermoplastic polymers provide substantially greater design freedom due to their easier mouldability.
However, the high diathermancy (i.e. transmittance for IR radiation) of transparent thermoplastic polymers leads to an undesirable temperature rise inside the vehicle under the influence of sunlight. As described by Parry Moon, Journal of the Franklin Institute 230, pages 583–618 (1940), most solar energy lies within the near infrared (NIR) range between 750 and 2500 nm, next to the visible range of light between 400 and 750 nm. Penetrating solar radiation is absorbed inside a vehicle, for example, and emitted as long-wave heat radiation of 5 to 15 μm. Since conventional glazing materials and transparent thermoplastic polymers in particular are not transparent in this range, the heat radiation cannot dissipate to the outside. A greenhouse effect is obtained. In order to minimise this effect as far as possible, the transmission of glazing in the NIR should be kept as low as possible. Conventional transparent thermoplastic polymers, such as e.g. polycarbonate, are however transparent in both the visible range and in the NIR, however. Therefore additives, for example, are needed that demonstrate as low a transparency as possible in the NIR with as high a transparency as possible in the visible range of the spectrum.
For applications in the automotive glazing sector a transmission in the visible range (LTA value) of at least 70% is prescribed in most cases. This value is defined in SAE J 1796 (issued May 1995).
The TDS value (solar-direct transmittance) according to SAE J 1796, issued May 1995, is used for efficiency of heat absorption. The value describes the percentage of solar energy that penetrates the sample and thus contributes to heating the interior.
Various heat-repellent systems having a low transmission in the NIR have been described in the literature. Surface coatings or paint systems are known on the one hand, and on the other hand there are also infrared-absorbing additives for transparent thermoplastic polymers. Since compositions comprising polymer and additive can be produced more cost-effectively, an NIR-absorbing additive would be desirable.
Examples of known NIR-absorbing additives include organic infrared absorbers, as described for example in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, page 1197 et seq. (1992).
Until now, however, no organic NIR absorbers have been known that on the one hand display suitable thermal and light stability and on the other hand reach a TDS of below 50% with an LTA of over 70%.
On the other hand, paint systems having NIR-absorbing particles consisting of indium tin oxide (ITO) have been described in the literature (e.g. in WO 00/14017). Depending on their composition and concentration, such additives likewise absorb in the NIR range. ITO particles that are embedded in an organic or inorganic matrix of a paint and that absorb NIR light effectively as well as being highly transparent in the visible range are known from JP-A 08011266, JP-A 0707482 and from JP-A 08041441.
The disadvantages of the paint systems described in the previous paragraph, however, are that they require a complex painting stage and that, incidentally, a sufficient quantity of ITO cannot be incorporated into the known paint systems without their becoming unstable.
In JP-A 070278795 polycarbonate is mixed with conventional ITO with the aid of a kneader. However, no reference was made to the transparency of the mixture. Conventional ITO produces cloudy composites with polycarbonate. This is not suitable for many of the desired applications in this case, e.g. for glazing systems.
When they are finely divided, conventional NIR-absorbing nanoparticles (nanoparticles should hereafter be understood to refer to particles having a size of less than 200 nm), which are invisible because of their small size, are suitable for inclusion in a paint system but not for incorporation into a thermoplastic polymer, since under conventional incorporating conditions the nanoparticles agglomerate, forming cloudy compositions due to light scattering at the agglomerates.
Until now, no thermoplastic moulding compositions with NIR-absorbing nanoparticles have been known that reach a TDS value of below 50% with an LTA value of over 70%.
Given the high cost of NIR-absorbing nanoparticles, it is further desirable to develop compositions that require as small as possible a proportion of these NIR-absorbing nanoparticles.
NIR absorbers which on the one hand absorb in a wide NIR range and yet at the same time demonstrate high transparency in the visible range of the electromagnetic spectrum, and which can be incorporated into transparent thermoplastic polymers without agglomerating, are desirable.